Diversity communications are a form of transmission and/or radio reception using several modes, using space and/or time to compensate for fading or outages in any one of the modes. In space diversity communications, two or more separate antennae receive the same radio signal which propagates simultaneously over several paths. If the paths are sufficiently different, independent propagation conditions could be expected. With time diversity communications the same path may be used, but the radio signal is transmitted more than once, at different times. There are other forms of diversity radio communications, using different frequencies, different polarizations, or different angle of arrival to provide diversity signals with different characteristics.
Diversity reception is a method of minimizing the effects of fading during reception of a radio signal. This is done by combining and/or selecting two or more sources of received/signal energy which carry the same intelligence but a different strength or signal-to-noise ratio in order to produce a usable signal. Diversity reception is widely and effectively used in commercial high-frequency installations. See Modern Dictionary of Electronics, Sixth Edition, Revised and Updated, Rudolf F. Graf, p. 281.
In terrestrial radio systems, the received signal usually consists of a major direct line of sight (DLS) component, and a few multipath (MP) components which are radiated in directions other than DLS and have been redirected by reflection, refraction or diffraction. Generally, these processes are unstable and the MP components' amplitude and phase fluctuate rapidly.
Usually, digital terrestrial radios operate with a dominant DLS signal and S/N of 30 dB or more. Under these conditions, data transmission performance is excellent. However, if the DLS component is attenuated by atmospheric defocusing or other unstable obstructions, and its amplitude becomes comparable to the MP components, the resultant received signal can experience rapid, intermittent deep fading. In terrestrial radios, such reception conditions are encountered during occasional anomalous signal propagation periods which can last for different time intervals with the probability of fading below a level of useful signal decreasing with increasing time. See U.S. Pat. No. 5,446,759. Such events are characterized by medium fades with occasional deep fade instants and intermittent data transmission problems in a form of short error bursts. See Digital Radio-Nature of Impairment, Diversity Signal Characteristics and Effective Designs, CRC Contract 67 CRC-5-2167, Nigrin and Polasek, 1996. It is during these events that the terrestrial radios experience most of their data transmission problems. In order to minimize these radio problems and achieve the required reliability, diversity antennae which provided suitable signals had to be used. The diversity signals were either combined in IF form using so-called maximum power combiners or were switched in the baseband. Unfortunately, both methods possessed flaws which prevented radio diversity transmission techniques from achieving their full potential. In maximum IF power combining, one diversity signal's phase was declared as the reference, and the other signal's phase was aligned to it. This asymmetrical approach fails when the reference signal experiences fading and its phase becomes unstable. See Digital Radio Outage/Fade Characteristics and Applicability of Current Prediction Techniques, CRC Report CRC-CR-94-002, Nigrin and Polasek, 1994.
Baseband switching, which was more expensive, was symmetrical and often performed slightly better. However, switching was done in milliseconds, initially based on signal power measurements which could not correctly identify which signal was experiencing error bursts at a given time. Later, many improvements were patented, such as a combination of fade and fade time rate or improvements based on channel estimation techniques, e.g. U.S. Pat. No. 5,351,274, or measurement averaging such as IF power, U.S. Pat. No. 5,325,403, or signal phase, U.S. Pat. No. 5,203,023. These techniques respond to average reception conditions and fail to make correct decisions when all (usually two) diversity signals experience intermittent error bursts. It is claimed that the diversity gain, i.e. the ratio of impaired seconds or severely impaired seconds of unprotected signals to impaired seconds of protected signals, increases with antenna separation and typically reaches hundreds. However, long term measurements showed that on bad radio hops, the gain is usually only tens. See Digital Radio Outage/Fade Characteristics and Applicability of Current Prediction Techniques, supra. These realistic diversity gain figure applies to 1975–85 radio designs which used only simple signal processing.
Currently, radios use complex signal processing, e.g. soft decision Viterbi demodulation, forward error correction (FEC) methods such as convolutional and block coding, etc., which translates into excellent data transmission performance improvements. It has been shown that if the conventional diversity techniques are applied to modern digital radios with sophisticated signal processing, the additional data transmission performance improvement is very small. See, Field Performance of a 128 QAM 155 MB(s) Sonet Digital Radio System, Boe et al, Globecom, 1991 at pp. 867–871. The diversity radio protected by both space and frequency diversity at the same time has not experienced any severely errored seconds (SES) compared to 7 SES of the unprotected radio. However, it recorded 92 errored seconds (ES) compared to 529 of the unprotected radio. Similar results obtained by other researchers raised questions about the need for diversity signal reception in view of its cost versus the actual gains. Boe's results document some weaknesses in the current digital radio designs. First of all, regardless of the signal processing complexity, radio systems which do not use diversity protection cannot provide a reliable data transmission performance when severe unstable multipath reception causes a bit error rate (BER) of 10−3 or worse. Second of all, conventional diversity techniques perform well when only one signal experiences error bursts, but are less effective during unstable multipath reception experienced by both diversity signals simultaneously. As a result, current diversity techniques do not seem to be adequate for mobile radio and other applications where radio waves propagate in close proximityto the ground and multipath signal reception is frequently encountered.
Other diversity communication techniques have been developed. See U.S. Pat. Nos. 4,384,358, 5,379,324, 5,402,451, 5,465,271, 5,487,091, 5,541,963, 5,566,364, 5,559,838 and 5,515,380. The U.S. Pat. No. 5,515,380 claims a method and a device which achieves significant performance improvements using diversity signal reception. The transmitted data are organized into blocks of bits. A block can be tens of bits long if parity bits are used or hundreds of bits long in case of FEC. Each block is augmented by adding error identification or correction bits, which upon signal reception and demodulation determine whether a given block contains errors. If a given block contains errors, the corresponding block received on the other diversity signal is hoped to be error free, in case of parity bit checking, or degraded to a lesser extent, in case of FEC block coding. The error-free, or better block, is selected to the output. However, when both diversity signals are affected by intermittent error bursts, the selection teelmique becomes ineffective. This is because parity bits ignore even number of errors and do not differentiate between single and a larger number of errors. FEC techniques use large data blocks which are likely to be similarly impaired by intermittent radio error bursts.
FIG. 1 illustrates a block diagram of a prior art space diversity communication system 10. The system includes a transmitter 12 which receives a baseband data input which modulates a carrier and transmits the modulated carrier from an antenna 14 through two separate transmission paths 16 and 18 to a pair of spaced apart antennae 20 and 22. These antennae are separated by a sufficient distance (e.g. a few wavelengths) to provide separate communication paths which are not subject to the same fading phenomena, such as Raleigh fading or other phenomena which degrade both transmissions 16 and 18 simultaneously. The received signal from antennas 20 and 22 is applied respectively to a pair of receivers 24 and 26. The output signals from the receivers 24 and 26 are applied to a combiner 28 which, as described above, functions to combine the output signals to produce a baseband output. The combiner 28 does not perform a comparison of respective streams of data units (e.g. bits) to choose and output individual data units as received from receivers 24 and 26 in circumstances where at least one difference in at least one data unit of a sequence of corresponding data units is identified and processing each data unit within the at least one difference of the sequence of corresponding data units to output data units having a higher probability of not being in error.
FIG. 2 illustrates a block diagram of a prior art frequency diversity system 30. A first transmitter 32 modulates a carrier of a first frequency with the baseband input and a second transmitter 34 modulates a carrier of a different frequency with the same baseband input. Antenna 36 broadcasts the respective modulated carriers 38 and 40 produced by transmitters 32 and 34 to a single antenna 42. The different frequency carriers are applied to receivers 44 and 46 which respectively process the data streams broadcast on carriers 38 and 40. Combiner 48 works in the same manner as combiner 28 of FIG. 2 and does not detect when at least one difference in at least one data unit of the sequence of data units transmitted by carriers 38 and 40 exists and thereafter processes each data unit within the at least one difference of the sequence of corresponding data units to output data units within each difference having a higher probability of not being in error.
FIG. 3 is a block diagram of a prior art radio transmitting system 50 which utilizes several techniques for improving digital radio data transmission performance. These techniques, which are all well known are provided by an outer forward error correction encoder 52 which adds error correction code to a data input. The output of the outer forward error correction encoder 52 is applied to an interleaver 54 which functions to rearrange the input data to result in a lower probability of consecutive transmission errors. The output of the interleaver 54 is applied to an inner forward error correction encoder 56 which adds inner forward error correction code. The output of the inner error forward error correction encoder 56 is applied to a modulator 58 which modulates a carrier with the composite of the data input which has been processed with outer and inner forward error correction code and interleaving. The output of the modulator 58 is applied to a transmitter 60 which amplifies the output of the modulator and applies it to antenna 62 which transmits the modulated carrier 64 to antenna 66. The output of the antenna 66 is applied to RF electronics 68. The output of the RF electronics 68 is applied to demodulator 70 which converts the signal back to its baseband form where it is subsequently processed by an inner forward error correction decoder 72 to decode the inner forward error correction code added by the inner forward error correction code encoder 56, by a deinterleaver 74 which reverses the effect of interleaving produced by interleaver 54 and by an outer forward error correction decoder 76 which performs the process of decoding the outer forward error correction code added by encoder 52. The data is outputted from the outer forward error correction decoder 76. The combination of outer and inner forward error correction code and interleaving produces a substantial reduction of errors caused by data transmission channel.